An N-channel MOS transistor comprises a drain region and a source region, both N-type doped, separated by a channel region covered with an insulated gate. Main electrodes are located on the drain and source regions. A positive voltage is applied between the drain and the source through the main electrodes. The flowing of a current through the transistor between the main electrodes is then controlled by a gate voltage, or control voltage, applied between the gate and the source. An advantage of the MOS transistor is that the control current which flows to the gate is close to zero.
When the control voltage is greater than a threshold voltage, the transistor is in a conductive state and a current flows through the transistor between the main electrodes.
When the control voltage is decreased from the conductive state, the MOS transistor switches to the off state as soon as the control voltage becomes lower than the threshold voltage. As long as the difference between the control voltage and the threshold voltage is smaller than a few hundreds of millivolts, there remains a leakage current, the transistor then being in a lightly blocked state. The intensity of the leakage current decreases as the control voltage decreases, in a way which depends on temperature. The variation of the leakage current according to the control voltage is characterized by a subthreshold swing value defined by the decrease of the control voltage, which causes a division by 10 of the intensity of the leakage current. At an ambient temperature close to 20° C., a MOS transistor has a subthreshold swing value greater than 60 mV/decade.
Due to the high subthreshold swing value, in order for the leakage current to disappear, the control voltage should be much lower than the threshold voltage, for example, distant by more than 500 mV from the threshold voltage. The transistor is then strongly blocked and no significant current flows through the transistor.
Similarly to the MOS transistor, a bipolar transistor comprises two N-type doped regions, in contact with main electrodes. The emitter and collector regions are separated by a base region. A positive voltage is applied between the collector and the emitter.
The bipolar transistor is in the on state when a control current is conducted from the base region to the emitter region. To achieve this, a control voltage greater than a threshold voltage is applied between the base and the emitter. The control current corresponds to the circulation of holes through the transistor. The presence of such holes enables a main current to flow between the main electrodes. The bipolar transistor then has a gain defined by the ratio of the main current to the control current. An advantage of the bipolar transistor is that it enables a particularly high main current to flow.
The control voltage is decreased to block the bipolar transistor. There remains a leakage current similar to that of the MOS transistor between the main electrodes of the bipolar transistor as long as the control voltage is close to the threshold voltage. In the same way as in the MOS transistor, the control voltage should be much lower than the threshold voltage so that the bipolar transistor is strongly blocked and that no significant current flows through the transistor.
There is a need for a transistor enabling to combine some of the advantages of MOS transistors and of bipolar transistors and to overcome all or part of their disadvantages.